Standard telephone networks utilize a plurality of signalling tones at various frequencies in order to implement control and supervisory functions, status messages, etc. For example, the well known "busy" signal is a repeating tone comprised of two particular frequencies. Similarly, the signal sent to the calling party when a called party's telephone is ringing is called "ringback".
The signal which is transmitted from the central office switch to the telephone in order to cause the telephone to ring is termed "ring tone". Ring tone is defined differently from country to country but is usually a relatively low frequency tone at a relatively high voltage. For example, in Germany, the specification for ring tone requires that a telephone detect tones anywhere between 23 and 54 Hz with a value of at least 30 volts rms while simultaneously rejecting frequencies in the range of 23 through 54 Hz having a value no greater than 22 volts rms. Additionally, the equipment must be capable of rejecting frequencies under 18 Hz and over 64 Hz if the amplitude of such signals is 75 volts rms or less. Finally, timing and cadence requirements dictate that detection occur within 200 ms.
In the United States, the ring tone specification requires that telephone equipment must be capable of rejecting signals no greater than 3 volts rms and of detecting, as valid ring tone, signals from 15-68 Hz at 40 volts rms or better.
Generally, tones of various frequencies may be detected through the use of digital signal processing. However, it is relatively difficult to detect these ring tones in software due to the following reasons. First the amplitude is much greater than that of normal tones received (e.g.; DTMF) through the telephone system. For example, the ring tone may be an AC signal of between 65 and 130 volts. In addition, in the United States, the ring tone includes a negative DC component, typically 48 volts, but which may run as much as 70 volts. Moreover, some countries require that in the worst case, the circuit must be capable of detecting the ring tone within half a cycle or less, making conventional digital signal processing algorithms unacceptable.
In some systems, signals from the telephone line are not passed to the system's digital signal processor until the equipment goes off hook. However, in systems which include Automatic Number Identification (ANI), signals from the telephone line are typically passed through a high pass filter and then fed to the digital signal processor, even when the telephone equipment is on hook. In these "on-hook filtered" systems, any signal below 300 Hz, including the ring tone signal, is greatly attenuated prior to the digital signal processor.
As a result of the above stringent requirements and resulting problems, ring tone detection is typically done through a hardware arrangement. One such ring detect circuit is described in the article "A Musical Telephone Bell" by Steve Ciarcia in Byte Magazine, July 1984. As described at page 128 of the Ciarcia article, the incoming signal is passed through a capacitor and on to a telephone bell, causing the bell to ring.
With the recent advent of voice processing systems, it has become prevalent to build computer equipment which is connected to the telephone line and which simulates a telephone handset. In such a case, the ring detect circuitry is built into a circuit board of the type typically installed into a personal computer or other type of circuit board rack. The problem with prior art ring detection techniques is that they are implemented in hardware and therefore can only be constructed to detect the ring tone signal in a particular country's telephone system. Thus, a manufacturer of such equipment must make numerous versions of its circuit card to ensure compatibility with telephone systems in various countries.
Moreover, in countries such as Germany, where stringent frequency requirements are present, it is difficult to ensure proper performance over the full range of operating conditions. More particularly, since the response of hardware analog components varies with temperature, humidity, etc., it is difficult to meet specifications under all conditions.
It is also noted that opto-isolation is required in these prior systems. Opto-isolation is utilized to ensure that the high ring voltage is not passed through to a user of the equipment.
As a still further problem, hardware components used for ring detection, such as diodes, optical detectors, etc., must be built onto the circuit boards used in voice processing applications, thereby occupying valuable space and increasing cost. Moreover, all of the hardware components are subject to failure, thereby decreasing reliability.
In view of the above, it can be appreciated that there exists a need in the art for a configurable ring detector that will occupy minimal circuit card space, while being capable of rapidly and reliably detecting very low frequency ring tones from the telephone line, which ring tones may be at different frequencies.